The present invention relates to a process and system for treatment of biochemical waste and more particularly to a process and system for anaerobic digestion of biochemical waste.
Recently, with the increase of equipments of secondary treatment of sewage and waste from the livestock industries, large quantity of surplus sludge is formed as by-products. Further, quantities of household garbage is considerably large. Accordingly, it is important to treat the biochemical waste with high effeciency without causing environmental pollution. The biochemical waste is now disposed of by incinerations, reclamations and dumping into the sea. However, it is known that secondary pollution is readily caused by these disposals. As local regulations on reclamations and dumping into the sea have been made strict, the inclinating method has now spread far and wide. However, this method involves various problems as to the treatment of smokes, bad smells and ashes formed by burning.
Biochemical waste such as activated sludges has been treated by anaerobic stabilization processes. It is known that the anaerobic stabilization processes are defined in terms of sequences involving two main reactions. More specifically, it involves an "acid fermentation" in which the molecular weights of the organic substances in the waste are reduced by anaerobic acid fermentation bacteria (acid formers, put refractive bacteria) and the substances are converted to volatile organic acids such as acetic acids, propionic acids and butyric acids, and a "methane fermentation" in which the so-formed organic acids are converted to methane gas by methane fermentation bacteria (methane formers, methane producing bacteria).
In conventional practice of the anaerobic stabilization processes, both phases of acid fermentation and methane fermentation co-exist within the same physical and chemical environment and process efficiency and control requirement are determined by the sensitivity and kinetic characteristics of the rate limiting phase. Accordingly, such a long period as thirty to fifty days is generally required for accomplishment of the anaerobic stabilization processes. However, the pollution preventing and energy saving characteristics of the anaerobic stabilization processes are recently reconsidered and many research efforts are being made to improve the fatal defect of these processes, i.e., a low treatment efficiency.
Recently, as a result of experiments using model wastes, it was proved that the above two fermentation phases can be separated from each other, and by this two-phase anaerobic stabilization process under optimum conditions, the treatment time can be remarkably shortened over the conventional process, which is written in "Developments in Anaerobic Treatment Processes" (Biotechnol. & Bioeng. Symp. No. 2 Page 85 to 106 1971, F. G. Pohland and S. Ghosh). In this two-phase process, the treated waste which has been subjected to the acid fermentation is separated into supernatant liquid containing organic acids and waste sludge by some solid-liquid separation units. The supernatant liquid is transferred to the methane fermentation stage and treated therein by methane formers.
The waste sludge separated after the acid fermentation is partly recycled to the acid fermentation stage and the remainder of the waste sludge is withdrawn from the treatment system.
We reproduced the two-phase treatment system, and found that the treatment efficiency can be improved but the gas yield is reduced by 10 to 30% as compared with the conventional process. We elucidated this cause and found that the reduction of the gas yield is due to the fact that parts of the waste sludge including cells of acid formers formed as by-products, separated after the acid fermentation is withdrawn from the system. In the anaerobic treatment of biochemical waste, it is important to improve the treatment efficiency, but in view of great demand for saving energy, it is desirable to increase the gas yield so as to utilize the digestion gases formed as by-products for heat and power sources of treatment equipments and utilize excessive gases effectively for other purposes. Further, as cells of acid formers include volatile organic acids such as acetic acids, propionic acids and n-butyric acids and typical bad smell producing components such as H.sub.2 S, it is not preferred to withdraw the waste sludges including the cells outside the system after the acid fermentation.